JDBC and SQLJ CIS 612 Spring 2009

JDBC JDBC is an API that enables database access from Java programs JDBC for DB access provides ◦ Portability across database servers ◦ Portability across hardware architectures JDBC drivers have been implemented for a number of DB servers (Oracle, SQL Server, DB2, Access, etc.)

JDBC Java Database Connectivity (JDBC) has been part of the standard Java standard edition since JDK 1.1 ◦ Current version is JDBC 3.0 (as of J2SE 1.4) ◦ JDBC classes are contained in the Java package java.sql ◦ A JDBC-ODBC bridge is included so that any ODBC data source available in the JVM host environment can be accessed

JDBC Drivers There are 4 types of JDBC drivers ◦ Type 1 JDBC-ODBC Bridge ◦ Type 2 Native API Driver ◦ Type 3 Network Protocol Driver ◦ Type 4 Native Protocol Driver Types 1 and 2, use platform specific code Types 3 and 4 are all-Java

Type 1

Type 2

Type 3

Type 4

JDBC Steps in JDBC applications ◦ Load JDBC classes (java.sql.*) ◦ Load the JDBC drivers  Class.forName(“oracle.jdbc.driver.OracleDriver”) ◦ Connect to the database  Connection conn = DriverManager.getConnection(url,userid,passwd)  ◦ Interact with DB using JDBC ◦ Disconnect from DB

JDBC Example /**********************************************/ /* A Simple JDBC Program (Section 5.2) */ /* Chapter 5; Oracle Programming -- A Primer */ /* by R. Sunderraman */ /**********************************************/ import java.sql.*; import java.io.*; class simple { public static void main (String args []) throws SQLException, IOException { try { Class.forName("oracle.jdbc.driver.OracleDriver"); } catch (ClassNotFoundException e { System.out.println ("Could not load the driver"); } String user, pass; user = readEntry("userid : "); pass = readEntry("password: ");

JDBC Example Connection conn = DriverManager.getConnection( "jdbc:oracle:oci8:"+user+"/"+pass); Statement stmt = conn.createStatement (); ResultSet rset = stmt.executeQuery ("select distinct eno,ename,zip,hdate from employees"); while (rset.next ()) { System.out.println(rset.getInt(1) + " " + rset.getString(2) + " " + rset.getInt(3) + " " + rset.getDate(4)); } stmt.close(); conn.close(); }

JDBC Example //readEntry function -- to read input string static String readEntry(String prompt) { try { StringBuffer buffer = new StringBuffer(); System.out.print(prompt); System.out.flush(); int c = System.in.read(); while(c != '\n' && c != -1){ buffer.append((char)c); c = System.in.read(); } return buffer.toString().trim(); } catch (IOException e) { return ""; }

The Connection Object A Connection represents a session with a particular database ◦ All SQL statements are executed and results returned in the context of a Connection ◦ A Java program can have multiple Connections to one or more DBs ◦ Default behavior is autocommit ◦ Connection object is used to send statements to the DB server

JDBC SQL Statement Classes There are three classes for sending SQL to the DB server ◦ Statement. Used for SQL statements with no parameters. ◦ PreparedStatement. Used when the same statement, possibly with different parameters is to be called multiple times. ◦ CallableStatement. Used for calling stored procedures.

Other Connection Class Methods public abstract void close() throws SQLException ◦ Immediately release the Connection’s database and JDBC resources public abstract void setAutoCommit (boolean autoCommit) throws SQLException ◦ Pass true to enable autocommit, false to disable it public abstract void rollback() throws SQLException ◦ Useful when autocommit is false public abstract DatabaseMetaData getMetaData() throws SQLException ◦ Get metadata from the DB

PreparedStatement Object PreparedStatement is compiled, can be reused, may contain parameters ◦ Example: PreparedStatement stmt = conn.prepareStatement(“insert into students values (?, ?, ?, ?)” ); … stmt.setString(1, id); stmt.setString(2, fn); stmt.setString(3, ln); stmt.setString(4, mi); stmt.executeUpdate(); /* executeQuery for Select */

ResultSet Class The result of an executeQuery is a single ResultSet object ◦ This class provides access to the table resulting from the query ◦ A cursor points to the current row (initially positioned before the first row. ◦ Use the next method to fetch the next row ◦ The current row’s column values are retrieved using the getXXX methods, where XXX is a Java type.  Pass either an index or the column name to specify which column

ResultSet Class Use the wasNull method to check whether the previously read column value was Null. Use getMetaData to obtain info like number, types and properties of the columns of a ResultSet

Errors and Warnings The SQLException class provides information about errors while accessing the db ◦ Contains the error message as a String object  Retrieve it using the getMessage method ◦ SQLState string identifies the exception according to the X/Open SQL spec  Retrieve it using getSQLState method ◦ Link to the next exception  Retrieve it using getNextException method

Errors and Warnings try { some JDBC statement to access the DB; } catch (SQLException e) { System.out.println(“SQL Exception caught!”); while (e!= null) { System.out.println(“Error Message = “ + e.getMessage()); System.out.println(“SQL State = “ + e.getSQLState()); System.out.println(“Error Code = “ + e.getErrorCode()); e = e.getNextException(); }

Errors and Warnings The SQLWarning class provides info about warnings generated during DB access ◦ Use getMessage to get the warning string ◦ Use getSQLState to get warning according to X/Open SQL Spec ◦ Use getErrorCode for vendor specific code ◦ Use getNextWarning for next warning generated

Scrollable ResultSet Since JDBC 2.0, ResultSet objects are now updatable and scrollable ◦ The Statement object must be created using the following Connection class method ◦ Statement createStatement(int resultSetType,  Int resultSetConcurrency) ◦ Where resultSetType is one of: TYPE_FORWARD_ONLY (default – old behavior), TYPE_SCROLL_INSENSITIVE (can scroll forwards and back, don’t see changes to ResultSet object in the DB), TYPE_SCROLL_SENSITIVE (changes are visible)

Scrollable ResultSet The resultSetConcurrency parameter takes one of the following values: ◦ CONCUR_READ_ONLY (default – old behavior) data is read only ◦ CONCUR_UPDATABLE – the Java program can make changes to the database based on the new methods and positioning of the cursor Create the scrollable ResultSet using the usual executeQuery method of the Statement object

Scrollable ResultSet Methods for cursor positioning include ◦ absolute(int row) ◦ relative(int row) ◦ first() ◦ last() ◦ previous() ◦ next() ◦ beforeFirst() ◦ afterLast()

More on Statements The Statement interface provides three different methods for executing SQL statements ◦ We have already seen executeQuery for statements that produce a result set ◦ The method executeUpdate is used to execute INSERT, UPDATE, DELETE or DDL statements  The return value is an integer indicating the number of rows affected by an update (0 for a DDL statement)

More on Statements The execute method should be used only when it is possible that a statement may return more than one ResultSet object, more than one update count, or a combination of these ◦ Possibly through executing a stored procedure call, or through executing a SQL statement which is unknown at compile time (e.g. use JDBC to implement SQL*Plus) ◦ Returns true if first result is a result set, false if it is an update count

CallableStatement A CallableStatement object provides a way to call stored procedures in a standard way for all vendor DBs ◦ Written using “escape syntax” (syntax the driver must translate for a particular DB) with one of two forms  Without result parameter – {call proc_name[(?,?, …)]}  With result parameter – {? = call proc_name[(?,?, …)]}

CallableStatement Passing IN parameters is done using the setXXX methods inherited from PreparedStatement ◦ pstmt.setLong(1, 12345); pstmt.setLong(2, 345); If OUT parameters are used, the JDBC type of each OUT parameter must be registered before execution ◦ cstmt.registerOutParameter(1, java.sql.Types.TINYINT); ◦ Then use getXXX methods to retrieve OUT parameter values  byte x = cstmt.getByte(1);